This invention relates to methods for detection and evaluation of fractures in earth formations by logging in a borehole penetrating the formations.
Fractures in earth formations are of major significance in the production of subsurface fluid resources such as hydrocarbons. In formations of low permeability and low porosity the potential production from a borehole into the formation is directly related to the number of open fractures. Secondary recovery of hydrocarbons (after production by the formation's inherent fluid pressure has been exhausted) involves the injection of fluids such as water to move hydrocarbons towards a producing well, and knowledge of the fractures in the formation in invaluable in predicting how injected fluids will behave. The strike, dip and density of subsurface fractures constitute valuable information in the search for hydrocarbon reservoirs in formations which have sustained complex deformation.
The current definitive source of information on fractures in the vicinity of a borehole is core samples obtained during drilling of the borehole. However, core sampling during drilling is an expensive procedure, and the subsequent analysis of the samples adds substantial additional expense and is time consuming. Furthermore, cores often cannot be obtained from formation intervals that are friable or densely fractured (precisely the sections of interest), and removal of the cores from the formation also removes them from the influence of the ambient downhole stresses, thereby altering or destroying stress-related characteristics in an unknown manner.
Borehole logging using measuring devices or sondes lowered into the borehole on an armored communications cable provides continuous measurements along extensive sections of the borehole under ambient stress conditions, at substantially less cost per foot than core analysis.
Various approaches to fracture detection and analysis using borehole logging techniques have been proposed. Thus an ultrasonic tool known as the borehole televiewer can be used to produce a visual representation of borehole wall reflectivity. However, it is difficult to produce good images on a reliable basis except in smooth-walled, nearly circular boreholes with a well-centered tool. Another, more recent, imaging tool measures electrical conductivity via arrays of buttons on two pads 90.degree. apart and pressed against the borehole wall. Although this tool provides significant advantages over the borehole televiewer, including a wide range of detectable fracture sizes and good vertical resolution, the electrical nature of its measurement precludes its use in boreholes containing oil-based (nonconductive) drilling fluids. In addition, as presently configured, only a portion of the borehole wall is examined in a single pass through the borehole.
Non-imaging logging techniques that have been investigated for fracture detection include: sonic logging; dipmeter measurements; resistivity, induction and spontaneous potential logs; density compensation logs; and natural gamma ray logs.
U.S. Pat. No. 4,575,828 to Williams describes a technique for distinguishing between total formation permeability and fracture permeability, involving a comparison of the ratio of tube wave amplitudes measured by a pair of spaced apart receivers and the difference in tube wave travel times measured by the receivers. However, the location of fractured zones is not accomplished using tube wave measurements but rather by means of measurements of shear wave attenuation.
In a paper presented by Paillet at the SPWLA 21st Annual Logging Symposium in July 1980 and entitled `Acoustic propagation in the vicinity of fractures which intersect a fluid-filled borehole`, full waveform logs including tube wave amplitudes were compared with known characteristics of the logged borehole. Although some correspondence between fractures and tube wave amplitude was observed, no procedure for analyzing tube wave data to detect fractures was presented.
European patent specification No. 127,548 describes techniques for using data from three spaced receiving transducers to formulate quantities representative of formation complex compressibility and loss parameters attributable to formation characteristics. Information concerning the presence or absence of fractures can then be derived from logs of these quantities and parameters.
U.S. patent application Ser. No. 686,127 in the name of Hsu et al, assigned to the assignee hereof, and a paper `A new method for fracture identification using array sonic tools` presented by Hsu et al at the 1984 SPE meeting, describe a method in which the energy of a selected wave component is calculated and displayed. When the selected component is a low frequency Stoneley wave, open fractures appear as a characteristic pattern on the display associated with Stoneley wave energy reductions caused by the fractures. Information on the fracture dip angle and fluid flow properties of the fracture may also be obtained.
Other techniques that have been reported include a study by Huang and Hunter, reported at the 51st annual SEG meeting in Los Angeles, 1981. In this work the occurrence of high-amplitude, low-velocity events in recordings of seismic energy originating from a shallow shot hole near the borehole was explained in terms of mode conversion at fractures leading to propagation of tube waves in both directions along the borehole away from the fracture locations.
There remains a need for a method of identifying and locating fractures by borehole logging, and of quantifying the characteristics of the identified fractures including their width.